free disk space as a size target

[dvc94ch]

It would be interesting to size the cache based on the amount of free disk space, so if the disk fills up the cache gets smaller to free up disk space.

[rklaehn]

Figuring out how much space you have would be out of scope for this library, so I guess this means that we would need a way to adjust size targets while the store is running, and not just on startup.